Automatic test equipment for checking printed circuit boards has long involved use of a "bed of nails" test fixture to which the circuit board is mounted during testing. This test fixture includes a large number of nail-like spring loaded test probes arranged to make electrical contact under spring pressure with designated test points on the circuit board under test. Any particular circuit laid out on a printed circuit board is likely to be different from other circuits and consequently the "bed of nails" arrangement for contacting test points in a particular circuit board must be customized for that circuit board. When the circuit to be tested is designed, a pattern of test points to be used in checking it is selected and a corresponding array of test probes is configured in the test fixture. This typically involves drilling a pattern of holes in a probe plate to match the customized array of test probes and then mounting the test probes in the drilled holes on the probe plate. The circuit board is then mounted in the fixture, superimposed on the array of test probes. During testing, spring loaded test probes are brought into spring pressure contact with the test points on the circuit board under test. Electrical test signals are then transferred from the board to the test probes and then to the exterior of the fixture for communication with a high speed electronic test analyzer which detects continuity, or lack of continuity, between various test points and the circuits on the board.
Some test applications require a different set of test points to be monitored during different tests. For example, in-circuit and functional testing require a different set of test points for each test. Functional testing applies electrical test signals to the input terminals of the circuit board and detects test signals on the output terminals of the circuit board. On the other hand, in-circuit testing applies and detects electrical test signals on test points located throughout the circuit board. Means for selecting different test points are required.
Various fixtures have been used in the past for performing both functional and in-circuit testing. One class of these fixtures sets the functional probes at a higher level in the fixture than the in-circuit probes. The printed circuit board is moved to engage the functional probes and not the in-circuit probes. After the functional test is completed, the printed circuit board is moved to engage the in-circuit probes. The circuit board under test is attached to a moving top plate, which moves down when a vacuum is drawn from a chamber below the top plate until stopped by a movable pin. In this stopped position, the circuit board contacts the functional test probes, but not the in-circuit probes. After functional testing is completed, the vacuum is released. As the fluid pressure in the vacuum chamber increases, the top plate moves up and away from the movable pin and disengages contact between the functional probes and the circuit board. The movable pin is then moved laterally to align it with a clearance hole in the top plate so that, when the vacuum is drawn during the in-circuit test, the movable pin moves into the clearance hole as the top plate moves down. Because the top plate is not stopped by the movable pin, the circuit board is free to move down to contact both the functional and the in-circuit test probes. This fixture requires the release of both vacuum and power when changing stages between functional and in-circuit testing. This release increases the cost of testing since test time is increased. The release of vacuum and power also has the disadvantage that, during troubleshooting of circuit boards that are failing functional tests, a failure occurring during functional testing may not reoccur during in-circuit testing after power is reapplied to the circuit board. The moving top plate and the probe plate are also susceptible to significant bowing from the moment force created by the atmospheric pressure on the circuit board that deflects the board around the movable pin which acts as a fulcrum for the moment force.
A second class of test fixtures places the circuit board in contact with both the in-circuit and the functional test probes simultaneously during in-circuit and functional testing. However, for this class, the in-circuit test probe differs from the functional test probe. A gap exists within the in-circuit probe that breaks electrical continuity from the circuit board to the test analyzer during functional testing. Electrical continuity is achieved during ir-circuit testing by closing the gap. In this fixture two vacuums are used for moving the top plate and the circuit board. The first vacuum draws the circuit board into spring contact with both probes but does not close the gap in the in-circuit test probe. In order to perform the in-circuit test, the second vacuum is drawn which further moves the top plate and the circuit board to close the gap in the in-circuit probe, thereby allowing test signals to be communicated from both the in-circuit and the functional test probes to the circuit board under test. This fixture has several disadvantages. First, all top level probes engage the circuit board during functional test. The upper portion of the in-circuit probe can act as an antenna which picks up stray signals and also as a capacitive load on the circuits on the circuit board. Second, two vacuum wells are required which add to the cost and complexity of the test fixture. Third, extra probes, additional drilling of holes, and additional parts are required, all of which increase the cost of the test fixture. Fourth, special probes must be used to provide the gap for the in-circuit probes, along with special receptacles for holding the probes in the vacuum wells. Finally, the use of special probes limits the dimensions of the spacing between test points, thereby precluding use of this style of fixture for new circuit board technology. As circuit board technology advances, circuit components are more closely spaced. The test points and probes for such circuits necessarily must be more closely spaced, requiring smaller probes and springs, which become more difficult and expensive to make.